This invention relates to a steering system for a non-track vehicle having a steering handle or a steering wheel, and at least one steered vehicle wheel operated thereby, with a variable steering transmission ratio existing between the adjusting movement of the steering handle and the adjusting movement of the steered wheels which changes with the steering angle. This invention also relates to a method of controlling a steering transmission ratio of a non-track vehicle
In the case of steering systems of passenger cars, the steering kinematics, as a rule, are constructed such that the steering becomes more direct with an increasing steering angle (the angle of the steered wheels with respect to a straight-ahead position). In other words, the steering transmission ratio between the adjusting movement of the steering handle and the adjusting movement of the steered wheels becomes smaller with an increasing steering angle. For example, in the case of a rack and pinion steering, this can be achieved by varying the base tangent length over the path of the steering rack.
In the case of conventional vehicle steering systems, the steering handle and the steered wheels are mechanically coupled with one another in a securely fixed manner in order to ensure a high safety under all conditions. U.S. Pat. No. 4,658,927 and U.S. Pat. No. 5,072,083 show known vehicle steering systems.
However, control systems without a mechanically fixed coupling are known in principle. For example, in airplanes, the horizontal stabilizers and landing flaps can also be operated by way of a control system without direct mechanical coupling. The position of the control handle operated by the pilot is sensed to generate a desired position signal having a value corresponding to a desired position of the horizontal stabilizers and landing flaps. The desired position signal value is read by a control system, which in turn sends out signals to motors which adjust the horizontal stabilizers and landing flaps to the desired position, as a function of a comparison between the desired position and the actual position. This concept, which is also called "fly by wire", has become so safe and developed that it is also used in passenger planes.
It is an object of the invention to provide an advantageous steering system for a non-track vehicle.
This and other objects have been achieved according to the present invention by providing a steering system for a non-track vehicle, and a method of controlling a steering transmission ratio of a non-track vehicle, having at least one steered wheel operated by a steering handle, the steering handle having a center position corresponding to a straight-ahead position of the steered wheel, the steering handle being movable between the center position and an adjusting position to move the steered wheel between the straight-ahead position and a steering angle, wherein a steering transmission ratio between an adjusting movement of the steering handle and an adjusting movement of the steered wheel is controlled according to a characteristic diagram at least as a function of the adjusting position of the steering handle or the steering angle of the steered wheel and of the vehicle speed.
According to the present invention, the steering transmission ratio between the adjusting movements of the steering handle and the steering angles of the steered wheels are varied at least as a function of the steering angle and the driving speed according to a characteristic diagram.
It is preferable that the steering handle and the steered wheels are operatively connected only by way of a controlled system which generates a desired steering angle value as a function of the position of the steering handle, and which uses at least one motor to adjust the steering angle of the steered wheels, the motor being controlled by the system to adjust the steering angle as a function of a comparison between the desired position and the actual position.
Since there is no direct mechanical coupling between the steering handle and the steered wheels, the steering transmission ratio between the steering handle and the vehicle wheels may be designed in a very flexible, parameter dependent manner, independently of the kinematics of the adjusting elements of the steered wheels. Furthermore, the present invention advantageously reduces the steering system space requirements in the vehicle, and allows increased flexibility in the design and layout of not only the steering system components themselves, but of other vehicle components, since a direct mechanical coupling between the steering handle and the steered wheels is no longer necessary.
According to one aspect of the present invention, the steering ratio between the steering handle and the steered wheels is advantageously controlled to be more direct, i.e., lower, in the case of larger steering angles as well as at low vehicle speeds.
In addition, the automatic change of the steering transmission ratio can easily be carried out such that, when the steering handle is held still, the respective adjusted steering angle of the steered wheels can also not change or change only in a delayed manner during changes of the parameters changing the steering transmission ratio.
In order to ensure that the straight-ahead position of the steered wheels always coincides with a center position of the steering handle, it is provided that, after a steering adjustment, the steered wheels will always reach the straight-ahead position exactly when the steering handle returns into the center position.
These and other objects, features and advantages of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.